Low-Temperature Combustion in Diesel Engines

Abstract

The growing energy demand for transportation has led to a shift towards eco-friendly combustion or improved diesel engines with increased efficiency, reduced emissions, and sustainability. Low-temperature combustion (LTC) aims to achieve controlled combustion, balancing optimal performance with lower NOx and SO₂ emissions. This chapter summarizes the recent trends in LTC strategies under further exploration such as fuel injection techniques, optimized air-fuel mixing, and accurate combustion phasing management, to discern existing literatures in extensive efforts to reduce flame stability and emissions. Subsequently, LTC faces challenges like stable ignition, precise control, and economical fuel choice. Liquefied biogas, methanol, bio-fuels, and thermo-physically enhanced biofuels are among the LTC diesel alternative fuels under investigation. Higher-octane fuels like biodiesels exhibited promising performance at low to medium loads, while natural gases and dual-fuel mode techniques seen promising choices for high-duty applications. Studies revealed that stakeholder collaboration could make cleaner fuel choices, meeting rigorous emissions rules while operating optimal LTC engines. Therefore, Future LTC research should focus on emission reduction, fuel flexibility, optimum performance at various working conditions, combustion stability, and accurate modeling and simulation.

1. Introduction

The world’s fastest-growing economies account for the majority of growth due to stronger infrastructure and commercial transportation; as a result, global transportation energy consumption, including developed economies, is predicted to ascend. There is an urgent need to increase transportation efficiency, reduce emissions, and promote sustainability on a global scale. Because transportation is an essential component of economic growth, the cost of commercial transportation solutions must be inexpensive while meeting customer expectations for total cost of possession, efficiency, dependability, resilience, and sustainability. Clean mixed controlled combustion engines are necessary for the transportation industry, as global transportation energy demand is expected to climb by 25% over the next two decades. The quest for sustainable transportation with the best environmental performance has led to rise in innovative combustion strategies for internal combustion (IC) engines. Diesel engines have well-established technology for their efficiency and durability currently facing challenges in meeting stringent emission regulations. Nitrogen oxides (NOx), particulate matter (PM), and volatile organic compounds (VOCs) are critical pollutants that must be controlled and managed because they have a negative impact on local and global air quality.

Low-temperature combustion (LTC) as its name indicates is a strategy to maintain a low-temperature combustion by reducing peak cylinder temperatures and pressures that involves a highly dilute premixed air-fuel charge before the start of combustion, particularly in internal combustion engines, with the purpose of a simultaneous reduction of ${\mathit{NO}}_x$ and soot emissions. Low-temperature combustion (LTC) strategy attracted plethora of new research significance as a promising alternative to conventional combustion for its benefits beyond reduced emissions such as lower peak pressures and temperatures, which enable reduced noise and vibration levels that enhance an overall driving experience, improved fuel consumption, improved thermal efficiency, and improved engine performance. By reducing formation of ${\mathit{NO}}_x$ and particulate matter (PM) pollutants emissions, LTC enables mitigation of adverse environmental impacts of diesel engines. One of LTC approaches is utilizing a highly dilute charge of premixed air-fuel mixture before the starting of actual combustion process aiming at more controlled and more homogeneous combustion process that results in enhanced emission characteristics and improved thermal efficiency. The conventional combustion relies on high pressures and temperatures that lead to formation of pollutants.

However, the LTC implementation in diesel engines presents its own challenges such as achieving stable ignition and precise control to optimize combustion phasing in highly dilute charge conditions that demand advanced engine management systems. Furthermore, efforts to characterize combustion behavior and reactivity in order to address issues of flame stability and further reduction of pollutant emissions have given rise to the development of advanced combustion technologies and diagnosis technologies. Advanced combustion technologies include partially premixed and homogeneous compression combustion, catalytic combustion, and carbon-free fuel combustion. The diagnosis methods include flow velocity, species, and temperature.

Additionally, the fuel choice plays a vital role to ensure a desired LTC combustion performance. This chapter consists of reviews of LTC in diesel engines from various published literature and presented in various subtopics as types and strategies of LTC, principles and challenges, combustion characteristics, experimental techniques, advanced technologies, consideration of transport sectors application, and future research directions.

2. Basic components of LTC, LTC strategy types, and their significance

Low-temperature combustion (LTC) strategies include:

1. Reactivity controlled compression ignition (RCCI),

2. Homogeneous charge compression ignition (HCCI),

3. Partially premixed combustion (PPC),

4. Dual-fuel combustion (DFC), and

5. Stratified charge compression ignition (SCCI).

Figure 1 shows a simplified schematic diagram with stratified and multiple fuel injection LTC engine. Generally, LTC combines homogeneous charge compression ignition (HCCI) and partially premixed combustion (PPC), which reduces NOx production by lowering combustion temperatures and starting the fuel spray earlier in the engine cycle.

Figure 1.

Simplified schematic diagram of a hypothetical LTC diesel engine setup (stratified and multiple fuel injection).

The key components of LTC to achieve an optimum LTC performance (efficient and clean combustion) comprise:

1. Modified shape and size combustion chamber to promote the desired combustion characteristics

2. Stratified fuel preinjector is used to charge small fuel amount in to prechamber in which a lean-air-fuel mixture is formed.

3. Multiple fuel-injection systems:

   1. Preinjectors: The introduction of multiple stratified fuel injectors charge small amount of fuel to aid homogeneous lean mixing of air-fuel

   2. Main injection: delivers bulk of fuel into main combustion chamber to ensure complete combustion.

   3. Post-injection: incorporating not more than a post-injection facility resulted in further NOx emission reductions.

4. Fuel pump and injection control unit: work in tandem to ensure precise regulation of fuel flow and injection timing based on various engine parameters.

5. Exhaust gas recirculation (EGR): Since the fuel gas analyses of LTC show that the exhaust gas has more unburned fuel content compared to the conventional diesel combustion (CDC), the EGR ensures taking back a portion of it back into the intake manifold.

6. Turbocharger: driven by exhaust gases compresses the intake air contributing to an overall engine efficiency enhancement.

7. Preheater: since exhaust gases temperature measurements show that LTC fuel gases have higher temperature than that of CDC, a preheater elevates the intake air temperature as well as reduce heat loss to enhance the combustion processes (mixing, complete combustion, and combustion stability) and improve combustion efficiency.

These facilities collectively contribute to ensuring LTC achieves improved fuel efficiency and reduced pollutants emissions. The main aim of LTC is to achieve low emissions while maintaining or improving thermal efficiency. Each strategy has its own advantages and disadvantages depending on the specific application and operating conditions.

These LTC strategies (summarized in the Table 1) can either be employed in the modern combustion systems in the diesel engines individually to exploit their corresponding benefits or combined to further enhance the system performance as shown in the hypothetical scheme Figure 1. HCCI, for example, is often possible by the integration of multiple fuel injectors and a pre-chamber, whereas SCCI increased system performance even further by achieving a leaner mixture with multiple ideal phased pre-injection time.

LTC Methos	Principle	Combustion controlled	Advantage	Disadvantage
Homogeneous charge compression ignition (HCCI)	The oldest premixed LTC strategy uses compression of a homogeneous air-fuel mixture until self-ignition. High exhaust gas recirculation (EGR) rates along with an early injection timing used.	adjusting the intake temperature (for ignition timing and combustion rate), EGR, and fuel injection timing. Other parameters include: Fuel properties (volatility and cetane number for reactivity), and air-fuel ratio (to ensure leaner mixture)	Low ${\mathit{NO}}_x$ and soot emissions	Risk of knock at high compression ratios, limited range, and difficult to control due to an indirect control.
Reactivity controlled compression ignition (RCCI)	Uses two different fuels: a low-reactivity fuel (LRF) that injected into the intake port using a port fuel injector (PFI) and a high-reactivity fuel (HRF) that is injected directly into the cylinder using a direct injector (DI). The fuel injection timing is adjusted for controlling the timing of the low and high-temperature heat releases (LTHR)/(HTHR) phases	Adjusting intake temperature, quantity, and LRF-HRF ratio for both fuels (for reactivity and advance combustion phasing, ignition timing, and combustion rate), fuel injection timing, EGR (to reduce ${\mathit{NO}}_x$ emission, peak temperatures, and pressures in the cylinder), and engine load (to regulate fuel injection rate and amount, ignition timing, and combustion rate)	better thermal efficiency and NOx-soot trade-off than CDC under a wide range of operating conditions	optimization of RCCI for different engine types and operating conditions not yet understood.
Partially Premixed Combustion (PPC)	An early injection timing for a partially premixed mixture of fuel-air in the cylinder before ignition. Involves delayed injection timing than HCCI resulting in stratified (less-homogeneous) air-fuel mixture that offers more control on injection timing, and more sequential auto-ignition.	Injection timing (delayed to enable more control). Other key combustion parameters are: air-fuel ratio, stratification level, and combustion phasing.	More operation range, less combustion noise, and reduced excessive maximum excessive pressure rise rates (PRR)	The high reactivity of diesel limits its range. Optimum operation under various conditions and fuel types is not yet fully understood.
Dual-fuel combustion	Like RCCI, dual-fuel combustion uses two different fuels, the primary fuel and another fuel, are used as secondary fuels to enhance combustion efficiency and reduce emissions. The dual-mode dual-fuel (DMDF) type where diesel and gasoline/natural gas are the primary and secondary fuels, respectively.	The combined adjustments of injection timing of both fuels (for the optimal combustion phasing and ignition timing), injection pressure (for fuel atomization & mixing quality), & intake air temperature (for reactivity).	Low emissions while improving thermal efficiency compared to conventional diesel engines.	Performance optimization for various operating conditions and fuel types is yet to be fully understood.
Stratified Charge Compression Ignition (SCCI)	Combines SI and HCCI, injecting small fuel near spark-plug that creates leaner mixture and while the rest of fuel is into intake port offering richer mix in the rest of combustion chamber.	Adjusting injection strategy and spark timing. Other parameters: air-fuel ratio, engine load, and intake temperature.	Stable and efficient combustion under certain conditions.	Complex process to optimize due to multiple parameters adjustment needs.
Conventional diesel combustion	Conventional diesel combustion involves injecting fuel directly into the combustion chamber at high pressure and temperature.	Adjusting fuel injection timing, injection pressure, EGR rate, and intake boost pressure.	Simplicity and reliability.	High ${\mathit{NO}}_x$ and soot emissions.

Table 1.

Comparison of various LTC strategies.

2.1 Objectives and scope

An energy surge due to spiking associated world population explosion and technology boom elevated concerns such as greenhouse gas emissions (GHG) and air pollution, climate change and health risks, and depletion of reservoirs over conventional burning of petroleum products to fuel power-essentials like engines. As a result, advanced combustion subjects such as fuel and combustion, alternative fuels, and diesel engine pollutants garnered an immense significance and unwavering attention from policy-makers, researchers, fossil fuel traders, engine manufacturers, and a plethora of other stakeholders. Low-temperature combustion (LTC) is considered as an engendered potential solution due to enhanced fuel efficiency, reduced emissions, and improved combustion characteristics. Yet, the underlying mechanisms, operational limits, optimization parameters, and effective advanced technologies of the LTC remain to be fully understood so that to unlock its practical potential across various industries operating under stringent emission regulations that make it particularly relevant for the diesel engine due to the LTC’s verified profile of minimizing the ${\mathit{NO}}_x$, PM, and beyond.

This chapter document aimed at summarizing the recent trends, challenges, and future prospectives in LTC diesel engines from various published and unpublished research and review literature and technical documents for the past 5 years. It starts with general principles of each LTC method, specific fuel characteristics requirement of the LTC along with comparisons frameworks. The second section describes challenges of implementing the LTC followed by experimental techniques used in monitoring and evaluation of the LTC diesel engines in the third section. The fourth section tries to address the LTC strategies along with advanced engine technologies from various renowned Scopus research and review databases followed by its various light to heavy-duty engines application in transport sectors in section five. The chapter concluded with the descriptions of the future research directions and perspectives of the LTC. Overall, by doing these, this chapter tries to answer the following academic questions about the LTC diesel engines:

1. What are the recent trends for LTC rail diesel engines to improve emissions, fuel efficiency, and durability?

2. What viable LTC implementation options and issues do the passenger car and commercial truck diesel engine manufacturers have?

3. What are the most promising hardware design modification and experimental validation for an LTC diesel engine?

4. What advanced engine control techniques optimize LTC diesel engine performance, emissions, and fuel economy?

5. LTC is a potential emission reduction technology, but how can it be implemented in diesel engines and locomotives given specific challenges and constraints of optimizing LTC for manufacturers?

3. Principles of operation and challenges in implementing the low-temperature combustion in diesel engines

Low-temperature combustion (LTC) refers to a series of combustion modes that can reduce soot and ${\mathit{NO}}_x$ emissions. It entails a lean air-fuel charge/preinjection before start of ignition (SOC), adjusting engine parameters such as start of injection (SoI) timing, air-fuel ratio, and injection pressure to reduce ${\mathit{NO}}_x$ and soot emissions, optimizing LTC performance by adjusting engine parameters such as SoI timing, air-fuel ratio, and injection pressure, and balancing combustion efficiency and heat transfer losses for optimal LTC performance. LTC optimization imposes a number of difficulties.

According to Olmeda et al. [1], the general combustion characteristics of LTC [2] are:

• A diluted air-fuel charge/preinjection before start of ignition (SOC).

• Setting the timing of the end of the main (EoI) of the direct fuel injection independent SOC to reduce NOx and soot emissions.

• Optimizing LTC performance is achieved by adjusting engine parameters such as start of injection (SoI) timing, air-fuel ratio, and injection pressure.

• SoI timings should balance combustion efficiency and heat transfer losses for optimal LTC performance. The optimal LTC performance with SoI timings saw a trade-off between combustion efficiency and heat transfer losses.

• The delayed start of injection (SoI) in diesel results in diffusive combustion with higher heat release profile with two peaks the first in the premixed phase and diffusive phase enhancing heat transfer.

3.1 Challenging and complex optimization process in LTC

The LTC is a diffusive combustion that maintains higher temperatures during the premixed phase, enhancing heat transfer when compared to conventional combustion. As stated in the last paragraph of Section 1.1.1, LTC can involve either a single strategy among the list in the table 0 or a combination of many, which involves a complex approach to optimize the system. Low-temperature combustion (LTC) modes, according to Dan et al. [2], are a series of combustion methods that can avoid the creation of soot and NOx emissions. Partially premixed combustion (PPC), partial fuel stratification (PFS), gasoline compression ignition (GCI), and reactivity controlled compression ignition (RCCI) are examples of LTC modes. According to the PDF, these tactics can avoid locally rich and high-temperature zones, which can result in soot and NOx emissions [1].

LTC optimization thus entails multiple and complex procedures, each of which presents its own set of obstacles, such as:

1. Multiple combustion dystem design parameters: chamber design modification, injection techniques, and air-fuel mixing.

2. Challenging fuel injection strategies: determination of the ideal timing, duration, and quantity of fuel injection to ensure balance between higher efficiency and reduced emissions.

3. Exhaustive combustion control: Controlling combustion parameters such as combustion timing and heat release rates impose difficulties in achieving consistent and steady combustion over various operating conditions and loads.

4. Complex emission control: Compromise between low ${\mathit{NO}}_x$ and particulate matter (PM) emissions and high combustion efficiency.

5. Meticulous combustion stability: regulation of air-fuel ratios and ignition timing to ensure stable combustion under a variety of operating circumstances, including idle, low load, and high load.

6. Incorporating sophisticated technologies like exhaust gas recirculation (EGR), turbocharging, and after-treatment systems into LTC brings new challenge of optimizing system interaction.

7. Difficulty of accurate modeling and simulation: realistic computational models and simulations to understand and optimize LTC processes is difficult to achieve

The most common conventional and renewable fuels being researched discussed in the next section along with their possible benefits and drawbacks considering their viable use in LTC diesel engines. Furthermore, extraction techniques from their ubiquitous fuel sources are also summarized in Table 2.

Fuel /Property	Diesel $C_{12}{H}_{24}$	n-Butanol $C_4{H}_9\mathit{OH}$	Gasoline $C_8{H}_{18}$	Biodiesel $C_3{H}_5{\left({\mathit{\text{OCOCH}}}_3\right)}_3$	Ethanol $C_2{H}_5\mathit{OH}$	Natural gas ${\mathit{CH}}_4$	Hydrogen $H_2$	Dimethyl ether $C_2{H}_{6}O$
Extraction	Petroleum extraction or fractional distillation	Fermentation of organic matter	Crude-oil refining or fractional distillation	Feedstock-oil extraction via pressing or chemical/solvent, then transesterification	Fermentation followed by distillation/purification	Deep-well-to-surface pumping of gas, followed by adsorption/purification of contaminants such as sulfur, ${\mathit{CO}}_2$, $N_2$ etc	Steam methane reforming Electrolysis Partial oxidation Biomass Gasification	gasification /syngas production preceding methanol synthesis and dehydration
Physical properties
flash point	65–88 ${}^{o}C$	35 ${}^{o}C$	40–45${}^{o}C$	130–160${}^{o}C$	16.6${}^{o}C$	NA	41${}^{o}C$	$-{42}^{o}C$
Density $\mathit{kg}/m^3$ stp	835–845	810–810.9	715–775	880–900	789	0.716	0.0899	2.02
Viscosity ${\mathit{mm}}^2/s$ stp	2–4.5	2.5	0.4–0.8	3.5–5.5	1.2	0.0172	8.76 $\times {10}^{-6}$	0.22
Boiling range	180–360 ${}^{o}C$	117–119 ${}^{o}C$	30–215${}^{o}C$	300–400${}^{o}C$	78.3–78.5${}^{o}C$	−82.6${}^{o}C$ @ 46 bar	0${}^{o}C$	$-{24.8}^{o}C$
Boiling point at stp	190–280 ${}^{o}C$	108.1${}^{o}C$	30–220${}^{o}C$	200–350${}^{o}C$	78.3${}^{o}C$	−161.6	−252.9${}^{o}C$	$-{24.9}^{o}C$
Sulfur cont. Ppm	$<15$	—	$<10$	$<10$	—	$<1$	—	—
chemical properties
cetane number	40–55	6–17	NA	50–60	8–15	NA	NA	NA
octane no.	NA	94	85–95	NA	100–105	85–110	130–140	55–60
Heat of combustion MJ/kg	42–46	27–29	44–48	39–43	26–30	50–55	120–142	28–32
Fuel sensitivity	—	—	3–12 RON	—	5–20	5–15	10–20	8–15
Ignition delay ms	2–5	2–8	—	2–6	—	—	—	1–2
LTCs: HCCI, RCCI, PPC, SCCI	High	Low	Low	Low	Low	NA	NA	NA
LTCs: DI benefits	High energy density and cetane no., good lubricity, robust systems	Low PM used as diesel blend, renewable, sustainable	Used for dual fuel as blend for low carbon alternative	Renewable, sustainable, low PM, and sulfur improved lubricity	Nearly similar to gasoline used as diesel blend	Low emission, abundant	Zero emission, high energy density	High cetane no., good cold start, renewable, and low emission
LTCs: DI Drawbacks	high emission, non-renewable	Low energy density, low ${\eta }_{\text{thermal}}$	Low cetane number, low energy density	High viscosity, low energy density	Low energy density, lower heating value	Low energy density, storage, and refueling infrastructure cost	Safety concerns and infrastructure cost refueling and storage	Low energy density, handling safety concerns

Table 2.

Comparison of fuel types for LTC: Extraction, physical and chemical properties, benefits, and drawbacks.

3.2 Fuel choice-extraction economy quandary and complex combustion characteristics

Various fuels tested in the LTC diesel engines with multifaceted outcomes due to fuel selection and imposed economy dichotomy, complex combustion process, and sophisticated combustion control and management requirements along with underexplored optimization methodologies.

Garcia & Boronat et al. [3] conducted research and review on the impact of low-temperature combustion-achieving techniques on diesel engine emissions for diesel and also employed in the experiment sustainable fuels such as biodiesel and renewable diesel. The result revealed that the use of alternative fuels can have both good and negative effects on the low-temperature combustion performance of diesel engines. Biodiesel, for example, can lower particulate matter emissions while increasing nitrogen oxide emissions, and renewable diesel can enhance combustion efficiency while increasing carbon monoxide emissions [3].

In the other studies, the authors proposed combining various alternative fuels, such as biofuels, methanol, and liquefied natural gas (LNG), to minimize emissions and satisfy environmental targets [4]. Using response surface methodology (RSM), regression model that involves identifying the optimal input factors in complex combustion process that results in desired engine performance. Elkelawy et al. [5] determined the optimal inputs of variables such as biodiesel percentage, alcohol percentage, retarded injection timing, injection pressure, piston geometry, and EGR percentage that will result in maximum brake thermal efficiency (BTE), minimum brake-specific fuel consumption (BSFC), and reduced emissions. Using response surface methods, it optimizes the input variables and output responses for a diesel engine running on diesel and low-grade coal-based generated gas. The equivalency ratio, compression ratio, and engine load were the input variables. The optimum input variables were 0.12 for equivalency ratio, 17.01 for compression ratio, and 12 Kg for engine load. Total of 3.54 kW for braking power (BP), 28.23% for brake thermal efficiency (BTE), and 0.38 Kg/kWh for brake-specific fuel consumption (BSFC). Despite this, emissions remained at 0.023% vol for CO, 4.2539 ppm for unburned hydrocarbon UHC, 0.9569 vol% for CO₂, and 9.6958 ppm for NOx emissions [5].

The specific important diesel fuel property requirement for optimal LTC operation include: viscosity, surface tension, and ignition tendency/cetane number. The four major challenges of LTC implementation with biodiesel despite its popular production and utilization that Wei et al. (2020) addressed in the book “Diesel and Gasoline” [6] can be summarized as:

1. Dilemma selection of an economic feedstock,

2. Complex specific fuel properties are prerequisites for LTC to ensure optimal performance, combustion control, and emission reduction in advanced engines systems,

3. Imposed significant temperature and pressure requirement and intensive methanol-demanding nature of the transesterification to reduce its viscosity, and

4. limited options of verified nonedible feedstock from which biodiesel can be derived and used without requirement of substantial alterations to conventional diesel engine.

It must be noted from the above table that the specific values vary depending on the fuel composition, blend or impurities, and extraction processes specifically for synthetic ones. The boiling range refers to the range of temperatures at which the highest to the lowest volatile constituents of the specific fuel evaporate where as boiling point refers to the specific temperature at which the fuel evaporates at the given standard pressure. The fuel with shorter boiling range and lower boiling is preferred because those enable better air-fuel mixing and volatility to ensure LTC. The other relevant fuel characteristics for LTC comprise cetane number-a measure of the ignition quality of diesel fuel, octane number-a measure of knock resistance of gasoline engine, heat of combustion, fuel sensitivity-the performance, difference between research cetane number (RCN) and motor octane number (MON) for gasoline fuels, and ignition delay. Generally speaking, fuels with higher values of heat release/heat of combustion, and cetane number with faster combustion or shorter delay, are considered to perform better in LTC. The final four rows illustrate the recent research modification and testing trend in LTC optimization attempts for various fuels. The three gaseous fuels in the three last columns with not applicable (NA) LTC application in the third row from the bottom are listed with benefits/drawbacks considering their properties along with potential LTC compression ignition application.

The experimental research by Jun et al. [7] on the combustion characteristics of ammonia, present it as a carbon-free fuel, describes its advantages as an alternative fuel with exceptional performance, durability, reliability, optimize combustion properties, and lower pollutant emissions. It did not go without challenges such as a lack of understanding of the ${\mathit{NH}}_3$ combustion characteristics, methods of combustion enhancement, and optimization of NOx formation in combustion, and problems such as low burning velocity in a combustion zone [7].

4. Combustion characteristics

The soot emission and delayed ignition are other major challenges in LTC diesel engine where unburned hydrocarbon leaves the exhaust and contribute to pollution. The relationship between the premixed mass and delayed ignition (DI) mass in a combustion zone is given in Eq. (1).

$$\begin{array}{c}\mathit{\text{PREi}}=\frac{m_{\text{premixed}}}{m_{\text{premixed}}+\mathit{MDI},i}\\ \mathit{MDI},i=m_{\text{premixed}}\left(1-\mathit{\text{PREi}}\right)\end{array}$$E1

where PREi is the PRE mass fraction of fuel in a combustion zone, mpremixed is the premixed mass, and MDI, i is the DI mass [7]. The experiment in ref. [7] revealed that

1. RCCI heat transfer has higher bulk temperature than diesel combustion.

2. RCCI combustion reduces heat transfer (an in-cylinder heat transfer loss) by 13% due to its shorter duration. The reduction is also associated to decrease in localized effect or uniformity of global (average temperature of entire combustion chamber) and local (temperature at the specific location within combustion chamber) temperatures.

3. The heat transfer losses for both low reactivity fuels (LRF): gasoline and 85:15 ethanol-gasoline (E85) blend exhibited similar trends with insignificant absolute differences, despite the exhaust gas temperature for the later being seen as higher due to the combustion process over longer period.

4. The E85 is seen potential LRF fuel for RCCI combustion to be cleaner and more efficient than diesel. However, engine settings optimizations such as SoI and EGR recommended to address the E85 drawbacks such as low heating value compared to gasoline, longer combustion duration, and high exhaust temperature.

5. Increase in gasoline fraction (GF) from 60 to 80% resulted in shorter period of combustion.

Ignition delay, therefore, is another undesired combustion characteristic in LTC diesel engines. It is related to the other important characteristics of combustion, equivalence ratio, the ratio of actual combustion air-fuel ratio to that of stoichiometric ratio. Considering multiple stages of chains of reaction, Eq. (2) is used to predict the delay time $\tau$ [2]:

$$\begin{array}{l}S=\frac{1}{\mathrm{\Delta }\tau },\tau =f\left(T,\varphi \right)\text{for}T=T\left(x\right)\&\varphi =\varphi \left(x\right)\\ \mathrm{\Delta }\tau =\frac{\partial \tau }{\partial x}\frac{\mathit{d\varphi }}{\mathit{dx}}+\frac{\partial \tau }{\partial T}\frac{\mathit{dT}}{\mathit{dx}}\end{array}$$E2

For the given zero initial concentration, the subsequent combustion, ${\varphi }_{\mathit{ox}}$ CFD estimator used showed that increase in temperature $T$ gradient with x-displacement of the stroke [8].

According to Antonio et al. [3], premixed low-temperature combustion strategies are used to investigate a way to reduce ${\mathit{NO}}_x$ and particulate pollutants directly during the combustion process, implying that it may have advantages over after-treatment systems such as selective catalytic reduction (SCR), though additional research may be required to fully evaluate the potential benefits and drawbacks of the technologies for diesel engines. Emissions testing under various engine loads and operating situations, as well as fuel efficiency tests, could be measurable and achievable approaches to evaluate the performance of low-temperature combustion systems [3, 9].

The equivalence ratio is used to predict predicting ignition and subsequent combustion, ${\varphi }_{\mathit{ox}}$ CFD estimator used

$${\varphi }_{\mathit{ox}}=\frac{2{\sum }_i\left(N_i{\eta }_{C,i}\right)+0.5{\sum }_i\left(N_i{\eta }_{H,i}\right)}{{\sum }_i\left(N_i{\eta }_{O,i}\right)}$$E3

for the $N$ number of moles of $C,H,\mathit{\text{and}}O$ in species $i$. According to Elkelawy et al. [5] with similar approach, adding nanofluid additives to blended diesel-biodiesel can reduce BSFC, reduce overall emissions, and boost BTE [5].

The spatial and temporal distribution of combustion within the engine cylinder during the combustion process are portrayed by combustion zone mapping shown in the Figure 2. The simulation data for typical fuel (n-Heptane) was obtained from Aneesh et al. [10] and presented here for conventional diesel combustion (CDC) and LTC side-by-side to enable comparison [10]. The greater fuel penetration with leaner mixture is seen for the CDC than LTC at entry/start of injection because combustion takes place at higher equivalence ratio. The combustion temperature at the fuel injection area was also observed to be higher but lower at the near exhaust (from left to right) for the CDC when compared to LTC. This implies faster combustion. The LTC, on the other hand, is seen to have fewer combustion-emissions intersection areas (reduced emissions). Relatively wider central combustion area (more uniform/homogeneous mixing) toward center and lower peak temperature for LTC account for reduced emissions.

Figure 2.

Combustion zone map comparison of CDC and LTC for n-heptane fuel in diesel engine.

Brake mean effective pressure and heat release rate are essential for combustion stability, extended combustion duration, and ignition delays. Eq. (4) is used in typical engine configuration to calculate mean effective pressure and heat release rate [11].

$$\begin{array}{lll}\overline{P}& =& \frac{{\int }_{{180}^{o}C}^{{180}^{o}C}\mathit{PdV}}{V_d}\\ {\eta }_I& =& \frac{{\int }_{{180}^{o}C}^{{180}^{o}C}\mathit{PdV}}{m_f{\mathit{LHV}}_f}\\ \mathit{HRR}& =& {\left.\frac{\mathit{dQ}}{\mathit{d\alpha }}\right|}_{\text{chemical}}-{\left.\frac{\mathit{dQ}}{\mathit{d\alpha }}\right|}_{\text{wall\hspace{0.17em}loss}}=\frac{1}{\gamma -1}V\frac{\mathit{dP}}{\mathit{d\alpha }}+\frac{\gamma }{\gamma -1}P\frac{\mathit{dV}}{\mathit{d\alpha }}\end{array}$$E4

Eq. (5) gives the relative heat release [12].

$$\begin{array}{lll}{\mathit{dQ}}_{\mathit{HR}}& =& g_c\times W_u\times \mathit{dHR}\left(\mathrm{a}.\right)\text{until\hspace{0.17em}this\hspace{0.17em}time}Q=g_c\times W_u\left(\mathrm{b}.\right)\text{Total}\\ \mathit{dHR}& =& {\mathit{dHR}}_i+{\mathit{dHR}}_{\mathit{str}}\left(\mathrm{c}.\right)\mathrm{Net}\mathrm{HR}\text{until\hspace{0.17em}present}{\mathit{HR}}_i=\mathit{HR}-{\mathit{HR}}_{\mathit{str}}\left(\mathrm{d}.\right)\mathrm{net}\mathrm{HR}\\ \mathit{HR}& =& \frac{Q\left(\alpha \right)}{g_c\times W_u}\left(\mathrm{e}.\right)\mathrm{HR}\text{until\hspace{0.17em}present}{g}_c,{\mathit{HR}}_i=\frac{U_i-U_{\mathit{ps}}+{\int }_{V_{a_{p}s}}^{V_t}\mathit{pdV}}{g_c\times W_u}\left(\mathrm{f}.\right)\text{Indicated}\mathrm{HR}\\ \mathit{HRR}& =& \frac{{\mathit{HR}}_i-{\mathit{HR}}_{i-1}}{{\alpha }_i-{\alpha }_{i-1}}\left(\mathrm{g}.\right)\text{Relative}\mathrm{HR}\end{array}$$E5

where $Q\left(\alpha \right)=$ the amount of heat released until the present,$g_c=$ the dose of fuel supplied in one engine work cycle, and $W_u=$ the calorific value of fuel in burnout fuel dose. $\mathit{HR}=$ heat release, $U=$ internal energy, the subscript $\mathit{str}=$ thermodynamic process cylinder stroke, $i=$ current injection process point, $\mathit{ps}=$ the beginning before the start of combustion, and $\alpha =$ crankshaft rotation.

Teyler et al. [13] used the brake-specific efficiencies: volumetric efficiency, combustion efficiency, and fuel efficiency from the energy conservation using the first law of thermodynamics on crank angle basis [14].

$$\begin{array}{lll}{\eta }_v& =& \frac{{\dot{m}}_{\mathrm{air}}\overline{R}{T}_{\text{intake}}}{{\mathit{NM}}_{\mathrm{air}}{P}_{\text{intake}}{V}_d}\\ {\eta }_c& =& 1-\frac{P_b}{{\dot{m}}_f\mathit{uel}}\left({\left({\mathit{bsQ}}_{\mathit{LHV}}\right)}_{\mathit{CO}}+{\left({\mathit{bsQ}}_{\mathit{LHV}}\right)}_{\mathit{THC}}+{\left({\mathit{bsQ}}_{\mathit{LHV}}\right)}_{H_2}\right)\\ {\eta }_f& =& \frac{P_b}{m_{\text{fuel}}{Q}_{\mathit{LHV}}}\\ {\eta }_{\mathit{th}}& =& \frac{{\eta }_f}{{\eta }_c}\\ {\mathit{\text{IMEP}}}_n& =& \frac{W_{\mathit{\text{in}}}}{V_d}\\ W_{\mathit{\text{in}}}& =& {\int }_{\mathit{ivc}}^{\mathit{ivo}}\mathit{PdV}+{\int }_{\mathit{ivo}}^{\mathit{ivc}}\mathit{PdV}=\frac{P_1+P_N}{2}+\sum _{i=2}^{i=N}\left(\frac{P_i+P_{i-1}}{2}\right)\left(V_i+P_{i-1}\right)\\ \frac{{\mathit{dQ}}_{\mathit{hr}}}{\mathit{d\alpha }}& =& \frac{{\mathit{dU}}_{\mathit{cv}}}{\mathit{d\alpha }}+\frac{{\mathit{dW}}_{\mathit{cv}}}{\mathit{d\alpha }}+\frac{{\mathit{dQ}}_{\mathit{ht}}}{\mathit{d\alpha }}+\sum h\frac{{\mathit{dm}}_{\mathit{cv}}}{\mathit{d\alpha }}\end{array}$$E6

Where $P_b$, $Q_{\mathit{LHV}}$, and IMEP are break power, lower heating value, and indicated mean-effective pressure, respectively. $W$ and $V_d$ are thermodynamic work and stroke volume, respectively. The indices ivc and ivo are inlet valve opening and closing, respectively. As shown in the Figure 3 (data obtained from ref [15]), LTC has higher mechanical efficiency despite having less thermal efficiency at TDC of the power stroke (beginning at ${90}^o$ cam angle) and lower mean effective pressure implying potential of achieving advanced combustion phasing. The higher mechanical efficiency also implies reduced specific base fuel consumption. Despite having lower combustion peak pressure and temperature see Figure 3, LTC tends to have higher heat release at the later combustion stages and therefore the exhaust leaves at relatively higher temperature. Regardless of having lower combustion efficiency of LTC as seen in Figure 4, the larger proportion of it being converted to useful work leads the LTC process to better performance. Overall, the low-temperature oxidation process results in an ultimate reduced emissions particularly, ${\mathit{NO}}_x$ emission.

Figure 3.

Comparison of combustion efficiencies and mean effective pressure (MEP) for CDC and LTC.

Figure 4.

Exhaust temperatures and emissions comparison of CDC and LTC.

The realistic consideration of alternative fuels for LTC gives their respective benefits and drawbacks, based on corresponding extraction methods from their feedstock or natural resource, physical and chemical qualities. The physical and chemical properties of such as flash point, density, viscosity, boiling range, sulfur content, cetane number, octane number, and heat of combustion are the most essential details in this text. Due to fuel selection, economy dichotomy, complex combustion process, and underexplored optimization approaches, the fuels’ corresponding testing in LTC diesel engines, produced diverse results. The optimum input factors for LTC are biodiesel %, alcohol percentage, retarded injection timing, injection pressure, piston shape, and EGR percentage, resulting in reduced pollutant emissions, maximum brake thermal efficiency (BTE), minimum brake specific fuel consumption, and maximum brake specific fuel consumption.

5. Experimental techniques for LTC analysis

The experimental techniques in LTC so far comprise:

1. Combustion diagnosis: to capture detailed information and combustion process such as high-speed imaging, laser induced-florescence (LIF), and particle image velocimetry (PIV).

2. In-cylinder pressure measurement: to analyze combustion process

3. Emission sampling and analysis

4. Optical access engines where transparent set-up allows for access through various optical diagnosis

5. Advanced fuel injection systems

6. Combustion chamber design

7. Engine calibration and control

8. Combustion kinetics and chemical analysis

According to BB, the investigated variables in single-cylinder diesel engines fuelled with diesel biodiesel and alcohols utilizing response surface methodology (RSM) are NOx, smoke emissions, and BSFC. The biodiesel percentage, alcohol percentage, retarded injection time, injection pressure, piston geometry, and EGR percentage are the data for input factors. The influencing factor for the variables was determined using ANOVA. Taguchi design was utilized to screen the elements and response surface method (RSM) was used to optimize the design [1].

Liu et al. [16] improved Method: Emission measurement methods for multicollinearity that relate speed/quality used to be based on three-stage OLSM of the estimated model of I/M test-based data to the vehicle emission with those measurement indicators are classified as:

1. Data emission measurement model based on driving condition,

2. Emission measurement model based on running state and emission measurement model based on road measurement.

More accurate regression model (stable: no collinearity: equal variance and residual normal distribution) or more fitting statistical model with profound relevant factors such as the driving mileage of the secondary trunk road, the service life of the vehicle, the load quality and the oil products, to estimate the vehicle emissions developed for the market-oriented road passenger emissions [17].

The technical report on experimental and analytical evaluation of combustion process parameters on FIAT 1.3 multijet SDE 90 HP engine equipped with Common Rail fuel supply system and electronically controlled solenoid injectors used rapeseed biodiesel or fatty acid methyl ester (FAME) and diesel oil Kierce University of Technology led by Dariusz et al. [12] to investigate the maximum combustion phenomena and reported that FAME exhibited slight $P_{\mathit{max}}$ than diesel oil at lower and medium loads despite the superior values at the first two subsequent stages of the maximum combustion [18].

The heavy-duty (HD) dual-fuel engine experiment by Zuev et al. [19] used single pilot injection with $\alpha$ varying 30 to 5${}^o$ crank angles at BTDC increased emission despite improving performance. A four-stroke heavy-duty V6 diesel engine YAMZ6566 with displacement 12 L, bore/stroke 130/140 mm, compression ratio 17.5, rated power 197 kW at 1900 rpm, maximum torque 1124 Nm in range from 1100 rpm to 1500 rpm used with fuel injection pressure of $P_{\mathit{max}}=1600$ bar, with $P_{max,\text{boost}}=1.5$ bar, at $20\pm 4^{o}C$ before pumping with the load instrumentation section made according to the standards of UN49-06 standard. In this standard, an in-cylinder pressure measurement of 25 and 100% loading at heavy-duty rpm of 1450 is recommended for HD dual-fuel engine. The double-shell piezoelectric pressure transducer with highest gauge of 250 bar is used for high pressure in the combustion chamber while intake and exhaust pressure are measured by another piezoelectric transducer up to 10 bar [19].

According to review information by Siti et al. [20], incorporating various sensor monitoring systems using advanced engine control strategies to corresponding engine parameters and adjusting fuel injection timing, quantity, and duration aided to capture enhanced fuel efficiency, reduced emission, and increased power output [20].

The Hunicz et al. [18] experiment used 99 percent hydrogenated vegetable oil from nonedible sources, with the remaining fraction of lubricant evaluated in the Neste’s commercial renewable diesel engine and compared to EN 590 diesel fuel, both of which were free of FAME. The heat release rate and mass fraction burnt results show that neither EGR nor MAP (manifold absolute pressure) had a significant impact on the combustion, with similar fast oxidation propagation beginning at TDC and incorporating both premixed and diffusion-controlled combustion with stratification/spraying. The heat release rate curve depicts a clear transition point with distinct fuel-type and mixing condition-dependent position toward the end of premixed combustion [18].

Based on experimental findings by Dempsey et al. [11], high-octane fuels methanol and di-tert-butyl peroxide (DTBP) were employed in the GM1.9 L engine, and robust mixing was guaranteed, resulting in the mean indicated pressure and gross indicated efficiency stated in Eq. (4) [11].

According to review by Dan et al. [2], employing fuel stratification analysis (FSA) methodology to establish combustion parameters, with its assumptions and validation using both reacting and nonreacting 3-D CFD simulations, is recommended. Premixed mass fraction and primary reference fuel (PRF) number formulae, assumptions of equal pressure in the cylinder, and validation results of probability density function (pdf) and fsa with reacting cfd pressure, temperature, and mass fraction are all used to calculate the (PDF) of various variables of interest [1]. The heat transfer characteristics experiment conducted by Olmeda et al. [1] on a premixed compression ignition (PCI) reactivity controlled compression ignition (RCCI) engine using light-duty 1.9 liters a single-cylinder diesel engine (SCE) with a geometric compression ratio of 17.1:1 and fixed swirl ratio (the ratio of tangential to axial velocities) of 1.4, equipped with a distributed thirteen 25-K type thermocouples [2].

6. Advanced technologies and strategies for low-temperature combustion

The popular recent technologies such as hydrogen fuel and electric battery vehicles require substantial electrical grid infrastructure along with large storage/transport/filling-station investment and their lifecycle emission is still high. The high octane number fuels (volatile renewable alcohol fuels and resistant to auto-ignition fuels) are expected to compete with those alternatives by undergoing performance enhancement and emission reduction efforts. Dempsey et al. [11] maintained rate of injection from diesel data while modifying the profile for density that the computational fluid dynamics (CFD) overpredicted the spike, which suggests the injection is during ignition delay period [11].

Advanced combustion refers to the study of combustion characteristics and flame stability aimed at reducing resultant pollutant emission to reverse the effect of global warming associated with the conventional combustion. It incorporates low-temperature combustion: refers to combustion maintained at lean-fuel conditions via premixing and preheating to enhance efficiency and reduce emissions, catalytic combustion: combustion at accelerated rate through addition of catalysts to enable LTC and reduce emission, carbon-free combustion: use of no or reduced ${\mathit{CO}}_2$ fuels or technologies that capture ${\mathit{CO}}_2$, and use of an accurate modeling and instrumentation: such as hot-wire anemometry and laser Doppler velocity meter for flow measurement, gas chromatography, and Fourier transform infrared spectroscopy for fuel gas species analysis, and resistance and bi-metallic strip thermocouples along with infrared thermography for temperature measurements [7].

A significant increase in research interest in advanced compression ignition (ACI) engines operating in the low-temperature combustion regime is seen due to their diesel-like thermal efficiency and low emissions. Thorough mixing and preinjection were used with a high octane fuels such as methanol, ethanol, and di-tert-butyl peroxide (DTBP) in reference to n-heptane and iso-octane. Low-temperature regime manifested by the changes in fuel ignition delay iso-contour and engine thermodynamic trajectory measured by parameters such as motor octane numbers (RON and MON) and octane sensitivity (OS), and effects of engine operating condition, e.g., intake conditions, global equivalence ratio and level of exhaust gas recirculation (EGR). Mingyuan et al. [21] found that the ignition occurs t = 1.49 ms, second-stage that thermal-chemical conditions do not deviate much from the initial values before the first stage ignition occurs employing the classical Zeldovich reactivity gradient theory to model advanced compression ignition engine in the low-temperature combustion regime in 1-D to explain the auto-ignition with thermal and concentration stratification using reacting CFD flow model and n-alkane fuel (n-heptune) [8].

A review by Dan et al. [2] on fuel stratification analysis (FSA) methodology, its assumptions, and its validation using both reacting and nonreacting 3-D CFD simulations employing methods of calculating the probability density function (PDF) of various variables of interest, such as premixed mass fraction and primary reference fuel (PRF) number (typically used to determine ignition delay of blend against reference), assuming uniform pressure in the cylinder, and validation results of pdf and fsa with reacting cfd pressure, temperature, and mass fraction [1].

The experimental research by Jun et al. [7] on combustion characteristics of ammonia as a carbon-free fuel, its advantages as alternative fuels capabilities with outstanding performance, durability and reliability, optimize combustion properties, and lower pollutant emissions, and challenges such as the lack of understanding of the NH3 combustion characteristics, methods of combustion enhancement, and optimization of NOx formation in combustion (page 2), and problems such as low burning velocities and high NOx emissions and methods for optimizing its combustion properties, because page 11 suggest the only existing numerical simulations using mechanisms usually overpredict or underpredict when compared with those obtained during actual combustion processing, which not yet done.

The experiment conducted on heavy-duty (HD) dual-fuel engine by Zuev et al. [19] investigated the fuel injection parameters such as spilt injection, pressure rise rate and heat release rate (HRR), and reduction in the emissions to improve the efficiency of start of injection while maintaining its gain of enhanced compression work and ${\mathit{NO}}_x$ emission reduction. An optimized injection strategy using ECFM-3Z models comprising: the breakup, evaporation, and spray-wall interaction models of 25.4% fuel preinjection with equally divided two crank angles (the second at 70 and the first at 10 ${}^o\alpha$) retarded with 5 ${}^o\alpha$ at bottom dead center (BTDC) pilots, 67.3% fuel a main injection along with a 7.3% post-injection that led to decrease in pressure rise rate and ${\mathit{NO}}_x$ emission by 20 and 10% with increase in PM by 5% developed compared to the default two stage injection with similar settings utilizing diesel fuel and biofuel (rape methyl ester). This was achieved by only monitoring one injection while maintaining the others on the CFD and verified by the experiment without automation [19].

Fuel stratification, according to Dan et al. [2], has sparked a lot of attention because of its potential for great thermodynamic efficiency and low fuel use. As a result of lower heat transfer losses and lean air-fuel mixes, the possibility for great thermal efficiency with minimal pollutant emissions exists. The novel fuel stratification analysis (FSA) method, which uses the first law of thermodynamics and assumes a homogeneous mixture in each cell of the computational domain, and t fuel is injected at a constant rate throughout the injection event, can determine the required distribution of fuel required to achieve an optimal heat release profile without extensive and costly experimental or modeling effort. The reacting CFD pressure, temperature, and mass fraction burned profiles were used as input conditions in the FSA analysis to predict ignition locations and the representative fuel distribution versus the nonreacting 3-D CFD simulation (used to generate PDFs of fuel distributions for various injection timings and initial conditions/to model fluid flow and heat transfer without considering a reaction) [1].

In the Polish standard (PN-EN 14214), the FAME ester is produced via transesterification of rapeseed oil triglycerides with methanol. This report also summarized how FAME’s high viscosity and density affect the injection and mixing processes, with the more oxygen content having the potential to contribute more spontaneous intensive ignition and complete combustion as fuel blends with 10, 20, and 50% volumetric content having delayed ignition with maximum heat release rates. The injection is electronically controlled by the crank load and speed and the air intake system is used with variable geometry turbocharger to improve the cylinder filling over the whole range of crankshaft speed. The change of fuel blend content is enabled by four valves attached to each cylinder controlled by two camshafts. Smart measurement systems are used to measure fast-changing values, with smart data acquisition system piezoelectric pressure transducer inside the cylinder and tensometric pressure sensor in the injection lines. The test was conducted in various configurations of load $10-60\mathit{Nm}$ and fuel composition of 5, 10, and 50% fuel dose with crankshaft rotation of 5, 10, 50, and 90% [12].

Shortened ignition time registered for with FAME blend obtained between 10 and 60 Nm with 5, 10, and 50% FAME composition. A significantly higher HRRs with $p_m\mathit{ax}$ wave recorded between only in the range 10-40 Nm and otherwise the same trend for crank rotation [12].

The ultra-low sulfur diesel (ULSD) experiment conducted using hotter premixed combustion was preceded by regulated pilot injection of at ${25}^o\le 20\%$. The use of ultra-low sulfur diesel (ULSD) in hot premixed combustion reduced the combustion noise while all pilot injections appeared beneficial with significant variations for medium 9 Nm load, and significant increase in particulate matter with increased combustion diffusion noticed without ${\mathit{NO}}_x$ reduction. For cleaner combustion, the larger load necessitated less premixing. For cleaner combustion, the larger load necessitated less premixing. The base-fuel consumption improved due to slower isobaric premixed diffusion (due to density and viscosity) with load for biodiesel at less than 9 Nm load, but with increased main combustion temperature and increased soot emission (PM/HC), ULSD demonstrated no dramatic peak temperature rise with fuel with load with improvements or no change in base specific fuel consumption (BSFC) in all multiple phase injections.

While a single injection trial demonstrated no benefit for biodiesel over ULSD. The higher the load, the higher the BSFC for the biodiesel, which was reported to be improved with delayed main injection. Regulating mean effective pressure discovered a relevant way to improve BSFC [14].

6.1 Advanced engine design and control

As the discussion so far mainly focuses on LTC, the recent automotive industries witnessed significant advancements in diesel engine technologies driven by improved fuel efficiency and reduced emissions. The LTC approach gained significant attention because the combustion process that occurs at low pressure and temperature enabled significant reduction in ${\mathit{NO}}_x$ and particulate matter emissions when compared to conventional diesel combustion.

The review by Siti et al. [20] revealed that some of the modifications such as using multiple injections (splitting the fuel into several smaller injections during the combustion process to reduce emission and improve fuel efficiency), high-pressure common rail injection systems (−delivers fuel to multiple injectors aiding precise control over the timing and amount of fuel injected into the engine to increase efficiency and reduce emission), and swirl chambers (to help fuel atomization and mixing with air to attain complete combustion) reported to have enhanced engine performance and reduced emission as well as enhanced fuel properties and overall combustion [20].

Norouzi et al. [22] developed a three-step multi-objective system model predictive control to accurately optimize complex thermo-kinetic reaction and non-linear turbulent flow inside the compression ignition internal combustion engine, incorporating various linear and semi-infinite non-linear models as well as real-time optimization, and achieved 40% compression ignition engine performance enhancement with 10–15% emission reduction, which was experimentally validated [22].

M. Elkelawy et al. [23] summarized the effect of nano-fluid additives on diesel/biofuel (B20-D80) blend and revealed that high heat release, high combustion, high thermal conductivity, and high oxygen content that percentage content increase with oxide of nano-graphene at 30, 60, and 90 ppm as well as reported on-going research in ${\mathit{Al}}_2{O}_3,{\mathit{TiO}}_2,$ and ${\mathit{CeO}}_2$ nanoparticle lead to a reduction in brake specific fuel consumption (BSFC), a reduction in all emissions, and an increase in brake thermal efficiency [5].

According to experimental studies by Antonio et al. [3], some of the most promising modifications to diesel engine fuel injection systems and combustion chambers to improve low-temperature combustion performance include reactivity-controlled compression ignition (RCCI), homogeneous charge compression ignition (HCCI), and dual-fuel combustion. These modifications have been validated through experiments and simulations. For example, it compared RCCI, HCCI, and CDC operation from low to full load in a diesel engine, while reviewing the impact of low-temperature combustion attaining strategies on diesel engine emissions for diesel and biodiesels [3].

Low-temperature combustion, therefore, relies on achieving controlled and homogeneous air-fuel mixing involving advanced fuel injection strategies, optimized air-fuel mixing, and precise combustion phasing control with key considerations such as fuel injection systems and air management.

7. LTC transport sectors applications in diesel engines

The model test for light-duty diesel transport (LDDT) and heavy-duty diesel transport (HDDT) by Mahesh et al. [24] revealed higher gaseous emission than standard and less for medium duty (MDDT) [17]. In an attempt to minimize the impact on greenhouse gases (GHGs) in short to medium term, the life cycle analysis (LCA) results revealed that natural gas claimed to be more reliable, affordable, and economical for HDDT and MDDT than diesel, while natural gas and LPG accessibility to fuelling station were still limited yet they offer cleaner combustion for vehicle applications, whereas more dense liquefied natural (LNG) offers more mileage over compressed natural gas (CNG), but demands complicated cryogenic infrastructure. Yet further reduction in carbon footprint possibility offered by switching to renewable natural gas without significant impact [25, 26].

Fats and oils pyrolyzed lower pour point, flash point, viscosity, and comparable calorific value to diesel. The diesel engine has continued to be the prime mover of the commercial heavy-duty road transport and marine vehicles since last century due to its high energy density and lengthy intervention, with the twice viscous fatty acid methyl ester (FAME) being the major 32% contributor, biofuel currently share of only 4% of the world transport energy demand [18].

Hansson et al. [27] conducted multi-criteria decision analysis for alternative marine diesel fuels and found that liquefied biogas, methanol, and bio-fuels as the most sustainable fuels. The MCDA, analytic hierarchy process (AHP) tool for managing complex decision problems and is used to find the optimal and most consensual solution by assessing interests and preferences alongside qualitative and quantitative information, approach involved identifying and weighting various criteria related to economic, technical, environmental, and social aspects of the different fuels. The authors also considered the influence of stakeholder preferences on the assessment. To gather data on stakeholder preferences, the authors conducted a survey of Swedish stakeholders involved in the maritime sector, including shipowners, fuel suppliers, regulators, and researchers. The survey responses were used to develop a set of scenarios reflecting different stakeholder perspectives. The authors suggest that a combination of different alternative fuels, such as biofuels, methanol, and liquefied natural gas (LNG), could be used to reduce emissions and meet environmental targets [4].

Ammonia has a far greater absolute minimum ignition energy than gasoline, making it safer to handle and carry, according to Jun et al. [7]. Furthermore, ammonia has the potential to be utilized as a fuel with performance comparable to traditional fuels such as gasoline and LPG. The driving range of an internal combustion engine using ammonia directly can reach 592 km, which is somewhat less than that of gasoline. Furthermore, because ammonia is a carbon-free fuel, it has the potential to minimize greenhouse gas emissions and reliance on fossil fuels [7].

Regardless of their recent growing concerns about fuel consumption, which necessitates extensive research into strategies such as delayed main injection, premixing, and further particulate-matter reduction, biofuels are expected to be major sources of transportation energy primarily for aviation, shipping, and heavy-duty road transport (trucks) rather than in light-duty vehicles [28, 29], where recent investments have encouraged pollution reduction, cleaner air, and optimum LTC combustion. According to Hunicz, hydrogenated vegetable oil derived from nonedible feedstock can fully utilize existing fuel infrastructure and is currently available as a stand-alone, drop-in fuel for diesel at over 500 filling stations in Europe and North America, with a 90% reduction in well-to-wheel carbon dioxide (CO₂) emissions when compared to diesel oil [18].

8. Future perspectives and research directions

While the LTC concept has been around for some time, the research continues to explore new dimensions and research perspectives with goal of further emission reduction, fuel flexibility, engine efficiency, combustion stability and control, and modeling and simulation. According to Antonio et al. [3, 30], more research is needed to thoroughly assess the possible benefits and drawbacks of various emission reduction methods for diesel engines, including low-temperature combustion. In addition, the report offered experimental data and analysis of various injection strategies for low-temperature combustion, which might be used as a starting point for future research [3, 9].

According to Hansson et al. [27], there is a rising interest in decreasing greenhouse gas emissions from the shipping industry, which has resulted in greater research and development of alternative fuels and propulsion technologies. It is suggested that the development and deployment of more flexible dual-fuel engines on ships is boosting fuel compatibility and, to some extent, lowering the risk of technology lock-in. Hansson et al. [27]. A dual-fuel engine, in general, can run on two or more distinct types of fuel, such as diesel and natural gas [3]. To accept the different fuels, the engine’s fuel injection system, ignition system, and other components may need to be modified. The particular adjustments required may vary depending on the type of engine and fuels used [4].

Due to scattered information in multiple-zone chemical kinetics-based chains of models for low-temperature combustion (LTC) engines, the reactivity controlled compression ignition recommended for accurate model and simulation of LTC and examined to fulfill the following requirements of LTC:

1. Premixed air fuel, and

2. In-cylinder lean charge

A HCCI proved to have less ${\mathit{NO}}_x$ emission than stoichiometric oxidation with localized low-temperature combustion that gave rise to multi-zone models. The ignition timing and HC/CO emission model of HCCI contains 291 species and 875 reactions but takes 3X as long as PRF20 model. A complete and accurate modeling requires coupling a set of submodels with multi-zone simulation that involves

1. Zero-dimensional gas exchange model: that captures the effect of variable valve actuation

2. Wall temperature model: enhancing the prediction of thermal stratification,

3. Simple rate-driven injection and fuel evaporation routines

4. Blow-by models or knock prediction functions

8.1 Alternative fuels for low-temperature combustion

On the other hand, Hansson et al. [27] conducted multi-criteria decision analysis and model estimation for alternative marine diesel fuels and revealed that the flexible dual-fuel engines can operate on two or more different types of fuel. In the context of the study, these engines are important because they increase the compatibility of different fuels with marine engines and reduce the risk of technology lock-in. The analysis report explained that the development and introduction of more flexible dual-fuel engines on ships is increasing fuel compatibility and reducing the risk of technology lock-in to some extent. This means that as more flexible dual-fuel engines become available, it may be possible for ships to switch between different types of fuels depending on availability, cost, and environmental considerations [4].

Using a high octane fuel with a combined prechamber injection and direct high-pressure injection offer stable combustion, improve efficiency (thermal/engine), and reduce emission; prechamber enables controlled combustion through prolonged mixing due to late oxidation during compression at top dead center (TDC), whereas high-octane fuels enable higher compression ratio and more advanced combustion control timing [11].

Hansson et al. [27] advise policymakers and industry stakeholders to collaborate in developing a supportive regulatory framework and infrastructure for alternative fuels. Finally, while implementing alternative fuel options, the authors underline the significance of taking stakeholder preferences into account and engaging in transparent decision-making processes. When implementing alternative fuel options, the authors underline the significance of taking stakeholder preferences into account and engaging in transparent decision-making processes. It should be noted that upgrading existing engines with dual-fuel capabilities is costly and may not be viable for all ships. However, when more versatile dual-fuel engines become available on the market, shipowners may find it easier to switch between fuel types without requiring extensive engine modifications [4].

Elkeway [5] proposes several promising future research directions in the field of diesel engine combustion using response surface methodology, with the goal of developing more efficient and environmentally friendly diesel engines that can meet increasingly stringent emission regulations while maintaining high levels of performance [5]:

1. Investigating the effects of different types and concentrations of nanoparticles on engine performance and emissions. Investigation of the impact of advanced combustion concepts such as LTC on engine performance and emissions.

2. Studying the impact of various injection strategies, such as multiple injections, on engine performance and emissions.

3. Exploring the use of alternative fuels, such as biofuels and synthetic fuels, in diesel engines to reduce emissions and improve efficiency.

4. Developing new models that can accurately predict engine performance and emissions under a wide range of operating conditions.

5. Investigating the impact of advanced combustion concepts, such as low-temperature combustion and homogeneous charge compression ignition (HCCI), on engine performance and emissions.

Furthermore, advanced combustion concepts, described in Table 1, powertrain integrations such as EGR, sustainability, and renewability are parts of future research direction. Overall, the LTC research will revolve around emission reduction, fuel flexibility, efficiency improvement, stability and control, and modeling and simulation as seen in the above literature.